Wireless communication is widely used by computing devices. Because of its widespread use, there is a risk that, at any time in any location, multiple computing devices will be present and transmitting wireless signals. Such transmissions may interfere with each other. To avoid such interference, transmission schemes have been developed to separate wireless transmissions by one computing device from wireless transmissions by other computing devices operating in the same location.
One approach for separating transmissions is to define time slots for the computing devices. Computing devices in the same location may use different time slots for wireless communication so that devices that could interfere with each other are not transmitting at the same time. This approach is sometimes called time division multiplexing.
Different protocols using different approaches to assign time slots are known. In some instances, time slots may be assigned to clients in a network. The time slots may be assigned in advance as part of the design of the network or may be assigned by a device acting as a network controller as devices connect to the network. Another approach is to allow individual devices in a network to select time slots by identifying time slots that are available for use.
As an example, 802.11 protocols use an approach called Carrier Sense Multiple Access (CSMA) to allow clients to select available time slots. Under CSMA, a device wishing to transmit on a channel “listens” on that channel before transmitting. If the device senses traffic on the channel, it waits a random interval and then listens again to determine if the channel is then available. The device may repeat the process of sensing and waiting a random interval until the channel is available. After multiple attempts, if the channel is not available, the client may determine that the channel is not suitable. The client may then attempt to communicate over a different channel or may take other corrective action.
Another mechanism for separating transmissions is called Frequency Division Multiplexing. With Frequency Division Multiplexing, different devices may be assigned different ranges of frequencies to use as channels for communication. The transmissions from different devices may be separated using frequency domain filtering even if multiple devices are transmitting at the same time. For Frequency Division Multiplexing to be effective, the channels used by the devices in the same location should not overlap, which can limit the number of devices that may communicate in one location at a time, particularly if portions of the frequency spectrum are unavailable because of interference.
A refinement on frequency division multiplexing is called Orthogonal Frequency Division Multiple Access (OFDMA). With OFDMA, an available range of frequencies is divided into multiple subcarriers. Each device may be assigned a set of subcarriers to use for transmitting information. The subcarrier assignments are said to be “orthogonal” in that the subcarrier assignments are made such that, to the extent practical. each device uses a unique set of subcarriers. To the greatest extent possible, no subcarrier is used by more than one device. There is no requirement, however, that the subcarriers be contiguous. As a result, subcarriers can be flexibly assigned and there is a high likelihood that devices in the same location can communicate without interfering with each other.